Non-spherical or “aspheric” surfaces are finding increasing use in optical systems because they offer an additional degree of design freedom. As the technology for creating aspheric surfaces improves, optical components with aspheric surfaces are being employed with increasing regularity in devices from cell phones to DVD players to video and still cameras.
With the increasing use of aspheric surfaces comes the increasing need to rapidly and accurately measure and characterize such surfaces. There are two main measurement techniques used to characterize aspheric surfaces. The first utilizes a wavefront-based measurement using wavefront interferometry. This measurement involves passing a wavefront through or reflecting a wavefront from the aspheric surface and then interfering the wavefront with a reference wavefront. The interference creates interference fringes that are representative of the deviation of the surface shape from the target aspheric surface shape.
The second technique utilizes a profile-based measurement using point measurement profilometry. This measurement involves scanning a non-contact optical measurement probe over the aspheric surface to accurately measure a gap between the aspheric surface and the probe. Because the path over which the measurement probe scans is well defined, the path provides a reference surface for measuring the deviation of the aspheric surface.
Each of these measurement techniques has advantages and disadvantages. For example, the wavefront-based measurement is very good at measuring the non-rotationally symmetric shapes and surface variations in the mid-spatial-frequency range but is not very good at measuring the rotationally symmetric surface component. On the other hand, the profile-based measurement is good at measuring the rotationally symmetric component but is not very good at measuring the non-rotationally symmetric component. This measurement is also not very good at measuring mid-spatial-frequency components because the scanning beam has a spatial extent (e.g., about 100 microns) that limits the measurement resolution. The profile-based measurement also has an inherent noise signature that masks certain surface errors.
Consequently, to obtain the best possible characterization of the aspheric surface, one needs to move the aspheric element from one measurement system to another measurement system and separately obtain the profile-based and wavefront-based measurements. The measurement data then needs to be collected from the two different measurement systems and combined to arrive at the final characterization of the aspheric surface. This takes an inordinate amount of time and effort.